1. Field of the Invention
The present invention relates to a lithium secondary battery, and more particularly, to lithium secondary batteries having improved reinforcing means for increasing the thickness of an upper end of the can connected to a cap assembly.
2. Description of the Related Art
In general, unlike primary batteries which are not rechargeable, secondary batteries are rechargeable. Secondary batteries are widely used in various applications including advanced portable electronic devices such as cellular phones, notebook computers and camcorders. Lithium secondary batteries operating at 3.6 V are rapidly developing because their operating voltage is approximately three-times higher than that of nickel-cadmium (Ni—Cd) batteries or nickel-hydride (Ni-MH) batteries which are widely used as power sources for electronic devices. Lithium secondary batteries also have excellent energy density per unit weight.
A lithium secondary battery generally employs a lithium oxide, as a positive active material, and a carbon material, as a negative active material. Such lithium secondary batteries may be classified as liquid electrolyte cells and polymer electrolyte cells based on the kind of electrolyte used. Lithium batteries using a liquid electrolyte are generally referred to as lithium-ion batteries, and lithium batteries using a polymer electrolyte are generally referred to as lithium-polymer batteries. Typically lithium secondary batteries are manufactured in cylindrical, rectangular and pouchy shapes.
FIG. 1 is a schematic exploded perspective view of a conventional cylindrical lithium secondary battery 10.
Referring to FIG. 1, the cylindrical lithium secondary battery 10 includes a battery unit 11, a cylindrical can 12 accommodating the battery unit 11 and a cap assembly 100 connected to an upper end of the cylindrical can 12.
The battery unit 11 has a positive electrode plate 13, a negative electrode plate 14 and a separator 15. The positive electrode plate 13, the negative electrode plate 14 and the separator 15 are sequentially disposed and wound in a roll, like a jellyroll. A positive electrode lead 16 is attached to the positive electrode plate 13 and a negative electrode lead (not shown) is attached to the negative electrode plate 14. An upper insulating plate 18 is installed in the upper end of the battery unit 11.
The cap assembly 100 includes a cap cover 110, a positive temperature coefficient (PTC) element 130, a safety vent assembly and a gasket 170. The safety vent assembly includes a safety vent 140, an insulating member 150 and a cap plate 160. The safety vent 140, the insulating member 150 and the cap plate 160 are sequentially stacked. The positive electrode lead 16 is electrically connected to the bottom surface of the cap plate 160.
A process of manufacturing the above-described conventional cylindrical lithium secondary battery 10 will now be described briefly.
The battery unit 11 is wound in a roll, like a jellyroll, and inserted into the cylindrical can 12. The upper insulating plate 18 is mounted on the upper surface of the battery unit 11. The can 12 has a beading portion 12a formed by beading. Next, an electrolytic solution is impregnated into the can 12 and the gasket 170 is then inserted into the resultant structure. The gasket 170 is mounted on the top surface of the beading portion 12a. The positive electrode lead 16, which is electrically connected to the positive electrode plate 13 of the battery unit 11, is welded to the bottom surface of the safety vent assembly. The negative electrode lead (not shown) is electrically connected to the negative electrode plate 14 of the battery unit 11 and is electrically connected to the can 12 by a predetermined process. Then, the PTC element 130 and the cap cover 110 are sequentially arranged on the safety vent assembly.
After arrangement of the cap assembly 100 is completed, the cylindrical lithium secondary battery is sealed by a crimping process. Then, a pressing process is carried out in order to maintain the overall height of the battery 10 at a constant level, thereby completing the manufacturing process of lithium secondary battery 10.
During the manufacture of the battery 10, a portion of the can 12 which is disposed above the beading portion 12a is intensely pressed. The pressure causes thermal deformation of the can 12. As a result, the tight seal between the cap assembly 100 and the can 12 is not ensured.